The people at the HS try to do their best, but no matter what it is still a guess. Sometimes the guess is simply what is popular at the time. I would never expect to find a purebred at a pound, yes they end up there, but never expect it.
The dog we have was from the pound, and he is very obviously a purebred English Setter. By chance, I happened to take him to the same vet that treated him when he was brought to WY from Texas by the original owner, so I ended up with his complete medical record - just not his pedigree. The pound won't pass them on to you when you adopt from them "because it would encourage the new owners to use the dog for breeding". I don't see how since he was neutered before even putting him up for adoption.